The present invention relates to measurement of current in a power transistor, and more particularly in a power MOS transistor, constituted by a plurality of parallel elementary cells.
FIG. 1 shows one of the most conventional circuits for measuring current in a power transistor MO. This circuit consists in arranging a measurement resistor R.sub.S between the power transistor terminal connected to voltage V.sub.S (the source) and the reference potential. The other terminal of the power transistor is connected to a high voltage through a load R.sub.L wherein flows a current I.sub.L, identical in that case to the current in the power transistor. The measurement of voltage V.sub.S across resistor R.sub.S gives an indication of the current value.
This circuit exhibits several drawbacks.
The first drawback lies in the fact that resistor R.sub.S is constantly in series with the power MOS transistor and causes an increase in the dissipated power which is not negligible since resistor R.sub.S must have a sufficient value so that the voltage across its terminals is detectable.
Another drawback of this circuit is that resistor R.sub.S is liable to vary with temperature or differ from one manufacturing batch to another in a way independent of the possible variations of the power MOS transistor characteristics. As a result, one obtains an inaccurate determination of its value and therefore of the current being measured.
In order to avoid the first drawback, prior art circuits such as the one illustrated by FIG. 2 have been used. In this figure, the power MOS transistor M0 is shown The upper terminal (drain) of transistor M0 is connected to a positive supply terminal through the load R.sub.L, the supply of which is to be controlled. The series connection of a transistor M1 and a resistor R.sub.S is arranged in parallel with the power transistor M0, the gates of transistors M0 and M1 being interconnected to voltage V.sub.GS. It is clear that the first drawback is avoided, but the second drawback, that is, the possible fluctuations of resistor R.sub.S independent of the fluctuations of the other circuit elements is not eliminated.